Ultra high SQ subs are going to be limited to linear motor design such as Adire xBl^2 or our own LMS drivers or JBL split gap, and some (not all) underhung designs or else, servo controlled subwoofers as Velodyne, Paradigm and a few others make, we also make a few servo's for our customers too. Anything else is just more of the same.
Its also imperative to EQ the driver after you install it into your box. You can spend a lot of money on a driver and end up having it sound bad because of a non linear frequency response. PEQ and filters are critical and can not be omitted at cost in any application, be it home or car audio.
good luck guys!